LTM4673 datasheet(62/132 Pages) AD

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LTM4683

Rev. 0

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APPLICATIONS INFORMATION

STATUS_MFR_SPECIFIC command is asserted, and the

ALERT_nn pin is pulled low. The fault can be cleared by

writing a 1 to the bit. If the user does not wish to see

the ALERT_nn pin assert if a PLL_FAULT occurs, the

SMBALERT_MASK command can be used to prevent the

alert.

If the SYNC signal is not clocking in the application, the

nominal programmed frequency will control the PWM

circuitry. However, if multiple parts share the SYNC_nn

pins and the signal is not clocking, the parts will not be

synchronized, and excess voltage ripple on the output

may be present. Bit 10 of MFR_PADS will be asserted low

if this condition exists.

If the PWM signal appears to be running at too high a

frequency, monitor the SYNC_nn pin. Extra transitions

on the falling edge will result in the PLL trying to lock on

to noise versus the intended signal. Review the routing

of digital control signals and minimize crosstalk to the

SYNC signal to avoid this problem. Multiple LTM4683s

are required to share one SYNC_nn pin in PolyPhase

configurations. For other configurations, connecting the

SYNC_nn pins to form a single SYNC signal is optional. If

the SYNC_nn pin is shared between LTM4683s, only one

LTM4683 controller can be programmed with frequency

output. All the other LTM4683s should be programmed

to disable the SYNC_nn output. However, their frequency

should be programmed to the nominal desired value.

INPUT CURRENT SENSE AMPLIFIER

The LTM4683 input current sense amplifier can sense

the supply current into the VIN01 and VIN23 power stages

pins using an external sense resistor, as shown in Figure 2

Simplified Block Diagram. The RSENSEn value can be pro-

grammed using the MFR_IIN_CAL_GAIN command.

Kelvin sensing is recommended across the RSENSE resis-

tor to eliminate errors. The MFR_PWM_CONFIG [6:5]

sets the input current sense amplifier gain. See the MFR_

PWM_CONFIG section. The IIN_OC_WARN_LIMIT com-

mand sets the value of the input current measured by the

ADC in amperes, which causes a warning indicating the

input current is high. The READ_IIN value will be used to

determine if this limit has been exceeded. The READ_IIN

command returns the input current, in Amperes, as mea-

sured across the input current sense resistor.

There is an IR voltage drop from the supply to the SVIN_nn

pin due to the current flowing into the SVIN_nn pin. To

compensate for this voltage drop, the MFR_RVIN will be

automatically set to the 1Ω internal sense resistor in the

Figure 2 Simplified Block Diagram. The LTM4683 will

multiply the MFR_READ_ICHIP measurement value by

this 1Ω resistor and add this voltage to the measured volt-

age at the SVIN_nn pin. Therefore, READ_VIN = VSVIN_

PIN + (MFR_READ_ICHIP • 1Ω). The MFR_READ_ICHIP

command is used to measure the internal controller cur-

rent. Using the READ_PIN command allows for reading

calculated input power.

PROGRAMMABLE LOOP COMPENSATION

The LTM4683 offers programmable loop compensation to

optimize the transient response without hardware change.

The error amplifier gain gm varies from 1.0mS to 5.76mS,

and the compensation resistor RCOMPn varies from 0kΩ to

62kΩ inside the controller. Two compensation capacitors,

COMPna and COMPnb, are required in the design, and the

typical ratio between COMPna and COMPnb is 10. Also,

see Figure 2 Simplified Block Diagram and Figure 27.

By adjusting the gm and RCOMPn only, the LTM4683 can

provide a flexible Type II compensation network to opti-

mize the loop over a wide range of output capacitors.

Adjusting the gm will change the gain of the compensation

over the whole frequency range without moving the pole

and zero location, as shown in Figure 28.

Adjusting the RCOMP will change the pole and zero loca-

tion, as shown in Figure 29. It is recommended that the

user determines the appropriate value for the gm and

RCOMPn using the LTpowerCAD tool.

CHECKING TRANSIENT RESPONSE

The regulator loop response can be checked by looking at

the load current transient response. Switching regulators

take several cycles to respond to a step in DC (resistive)

load current. When a load step occurs, VOUT shifts by an

amount equal to ∆ILOAD • ESR, where ESR is the effective

LTM4673 Datasheet(PDF) 62 Page - Analog Devices

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